feat(admin): always-on dashboard widget + default retro days = 0

- Dashboard widget is now registered unconditionally. Header + content
  adapt: shows '⚠️ ошибки доставки' only when failed > 0, otherwise
  plain title with queue state (pending / sent-24h / failed).
  Pilot feedback: the conditional widget felt like the plugin was
  missing — users expect a persistent status readout.
- Retro default window changed from 7 days to 0 (no limit). Most
  pilot uses were 'dump everything since install', so 0 is the more
  useful default. User can still narrow to N days in the form.
- Spec §10 + §14 updated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

admin/views/settings.php

@@ -83,7 +83,7 @@ if (isset($_GET['purged']))        $notice = '<div class="notice notice-success"
             </tr>
             <tr>
                 <th>Только за последние</th>
-                <td><input type="number" name="retro_days" value="7" min="0"> дней <span class="description">(0 = без ограничения)</span></td>
+                <td><input type="number" name="retro_days" value="0" min="0"> дней <span class="description">(0 = без ограничения)</span></td>
             </tr>
             <tr>
                 <th>&nbsp;</th>